C.sub.60 is the prototypical member of a family of soccer ball-shaped, aromatic, cage molecules, called fullerenes, which comprise varying numbers of covalently-bound carbon atoms. Macroscopic quantities of fullerenes in solid form were first disclosed by Kratschmer, et al., Nature, 1990, 347, 354, who called the material "fullerene". The mass spectrum, x-ray diffraction data, and infrared and ultraviolet/visible spectra for this material indicated that it contains a mixture of fullerenes, with C.sub.60 the predominant species and C.sub.70 present in appreciable amounts. Fullerenes comprising 76, 84, 90, and 94 carbon atoms have also been reported.
Hebard, et al., Nature, 1991, 350, 600, have reported that potassium-doped C.sub.60 is a superconductor having a superconducting transition temperature (T.sub.c) of 18 K. Also, Rossensky, et al., Phys. Rev. Lett., 1991, 66, 2830 have reported that rubidium-doped C.sub.60 is a superconductor with a T.sub.c of 28 K. Holczer, et al., Science, 1991 252 1154, have confirmed and extended these findings to include the superconducting properties of C.sub.60 doped with a variety of alkali metals. Holczer, et al. indicate that a single, potassium-doped C.sub.60 phase --K.sub.3 C.sub.60 --is the superconducting phase, with a T.sub.c of 19.3 K, and that neither under- nor overdoped phases are superconducting. In addition, Ebbsen, et al., Nature., 1991, 352 222, have disclosed ternary fullerenes doped with cesium and rubidium having T.sub.c of 33 K.
Despite the apparent utility of metal-doped fullerenes, there presently exists no simple, stoichiometrically-controlled method for their synthesis. Typically, a procedure such as disclosed by Holczer, et al. is employed, wherein a sample of C.sub.60 is treated directly with a measured portion of the desired metal to give a stoichiometry M.sub.3 C.sub.60. However, since it is generally desirable to use a minimum amount of the relatively expensive C.sub.60, the amount of metal required typically is quite small and difficult to accurately dispense. Thus, given that the superconductivity of metal doped fullerenes is sensitive to the exact degree of doping, it is difficult to produce materials of controlled superconductivity using procedures such as disclosed by Holczer, et al. Wang, et al., Inorg. Chem., 1991, 30 2838 and Inorg. Chem., 1991, 30 2962, have disclosed a procedure utilizing solution synthesis, but this technique also appears to suffer from problems with stoichiometric control.
Lieber, et al., Nature, 1991, 352, 223, have disclosed a procedure which provides greater degree of control over the stoichiometry of the doping process. This procedure requires the employment of binary alloys comprising both heavy metals and alkali metals wherein the heavy metal serves to decrease the reactivity of the alkali metal. However, Kraus, et al., Z. Phys. B (submitted) have indicated that the heavy metal may actually be deleterious to the superconductivity of the doped material.
Accordingly, it is a one object of the present invention to provide compositions of metal-doped fullerenes comprising a relatively high percentage of the superconducting phase. It is a further object to provide a simple yet stoichiometrically-controlled method for the preparation of such compositions.